A cryogen driving system comprise a cryogen drain port (112, fig 2) and a first self-pressurising gravity-fed cryogen storage unit 118 for receiving a cryogen and is in fluid communication with the drain port. A second self-pressurising gravity-fed cryogen storage unit 120 for receiving the cryogen in fluid communication with the drain port. The drain port may be in communication with separate inlet ports 114, 116 that respectively feed the storage units. The first and second storage units respectively alternate between a filling and a purging state. The first and second storage units may be in fluid communication with a pressurizing unit 126 which may be pumpless and may be a vaporiser that uses cryogen from the storage units to pressurise said storage units. A flow controller (202, fig 3) may be used to toggle between the alternating filling and purging of the first and second storage units using valves 122, 124. Inhibiting the filling of one tank occurs when it contains a predetermined level of cryogen. A
